2 Controller tuning A systematic-adjusting procedure of the controller parameters to obtain a desired performance of the control system PID control tuning Introduction (1) It is a matter of selecting the right mix of P, I, and D action to achieve a desired performance 3 Performance criteria for closed-loop systems Stable Minimal effect of disturbance Rapid, smooth response to set point change No offset No excessive control action Robust to plant-model mismatch Trade-offs in control problems Set point tracking vs. disturbance rejection Robustness vs. performance Introduction (2) 4 2

3 How do we know when it s tuned? Introduction (3) The process didn t blow up The process measurements stay close enough to the setpoint Boss says OK, and you can go home You buy a new controller which has different PID algorithm 5 Introduction (4) The Problem We have the knowledge about the effect of each PID modes to closed-loop response But, from what values of P, I and D modes we would pick to start to tune? The Solutions If you have tuned the process before, use slightly different values of the old PID controller parameter If the results are still not satisfy you, use a PID controller tuning method that we will learn just in a moment that is most suit to your process control application. Keep watch on 6 3

4 General Tuning Procedure Introduction (5) Before tuning, FAMILIARIZE with the OPERATION RISK Get help with experienced operators, explain your work to him and tell him that NO PERMISSION IS REQUIRED if their intervention is NECESSARY to save the loop if things go wrong 7 Introduction (6) Precaution: All kinds of tuning method should be used for initial setting and fine tuning should be done!!! 8 4

5 Cohen-Coon Coon Open-loop Tuning Method (1) Proposed in 1953 by G. H. Cohen and G. A. Coon 1 Main principles: The process output is affected not only by the dynamics of the main process but also by the dynamics of the measuring sensor and final control element They observed that the response of most processing unit to an input change had a sigmoidal shape CO PV time time 1) G. H. Cohen and G. A. Coon, Theoretical Consideration of Retarded Control, Trans. ASME,Vol. 75, pp. 827, Cohen-Coon Coon Open-loop Tuning Method (2) Main principles: (contd.) The sigmoidal shape can be adequately approximated by the response of a first order system with dead time CO T d A B S time Actual response Approximate response time G fpm = PVm ~ K e -T d s CO τ s + 1 where K τ T d = = = B A B S, S is the slope of the sigmoidal response at the point of inflection time elapsed until the system responded, 10 5

8 Ziegler-Nichols Open-loop Tuning Method (4) 6. Once the value of process parameter are obtained, the PID parameter can be calculated from the following table Controller P I m D 1 τ P only - - K T d 0.9 τ 0.33 T d PI - K T d PID 1.2 K τ T d 2 T d 0.5 T d 15 Ziegler-Nichols Closed-loop loop Tuning Method (1) Proposed in 1942 by J. G. Ziegler and N. B. Nichols of Taylor Instruments (now part of ABB instrumentation in Rochester, N.Y.) Also known as continuous cycling or ultimate gain methods It has been developed for use on delay-followed-by-firstorder-lag processes It has been refined for other specific process control objectives 16 8

9 Ziegler-Nichols Closed-loop loop Tuning Method (2) The Procedure 1. At the controller, select proportional-only (P-ONLY) control, i.e. set P to the lowest value (PB to the highest value) and I m to infinity (I r to zero) and D to zero (smallest possible influence of the controller) 2. Adjust the controlled system manually to the desired operating point (start-up control loop) 3. Set the manipulated variable of the controller to the manually adjusted value (reset bias b) and switch to automatic operating mode 4. Continue to gradually increase P (decrease PB) until the controlled variable encounters harmonic oscillation. If possible, small step changes in the setpoint should be made during the P adjustment to cause the control loop to oscillate 5. Take down the adjusted P value as critical proportional-action coefficient P crit 17 Ziegler-Nichols Closed-loop loop Tuning Method (3) The Procedure (contd.) 6. Determine the time span for one full oscillation amplitude as t crit, if necessary by taking the time of several oscillations and calculating their average 18 9

14 Lambda Tuning s Rule of Thumb: Lambda Tuning (4) Integral time should no be smaller than the process time constant Level control oscillating? Remove nearly all integral action Poll time should be less than one-tenth the process time constant Filter time constant should be less than one fifth the process time constant Closed-loop time constant is usually greater than the process time constant 27 Visual Loop Tuning (1) Problems The loop is unstable (or apparently so) The loop is sluggish in response to upsets or setpoint changes How to improve the performance of a loop by using NO algebra? SP SP SP Apparently unstable response Good response? Sluggish response 28 14

15 Apparent Instability The loop oscillates Because of excessive feedback, or Of being perturbed periodically by another process Visual Loop Tuning (2) Procedure: Put the loop in manual (if it is safe to do so) In manual mode, the process appears to settle down poorly tuned Tuning problems: Is the oscillation caused by too much or too little gain/integral/ derivative or their combinations? SP 29 Apparent Instability (contd.) Tuning procedure: Self-regulating Processes Visual Loop Tuning (3) If the value of I m (min/repeat) is less than half of the oscillation period τ SP First, Increase the value of I m If the value of I m (min/repeat) is longer than the oscillation period, it is safe to decrease the gain (P) τ SP Apparently unstable response Increasing the value of I m (min/repeat) Lowering the gain (P) Non Self-regulating Processes Use the longest value of I m (min/repeat) as much as possible or completely remove the integral action. If the problem persists, then lowering the gain (P) 30 15

16 Visual Loop Tuning (4) Sluggish Response Common causes: The loop usually has no derivative action The value of I m (min/repeat) is long relative to the process response time The value of gain is too low SP 31 Sluggish Response (contd.) Tuning procedure: 1. Adjusting Gain Set the I m as longest as possible and set D to zero. Visual Loop Tuning (5) Place the controller in manual mode, then step out the CO at a reasonable value Immediately put the controller back in auto mode. Watch the process response to know what the controller action does Repeat the process until we get one cycle of process output swinging Swinging more than one cycle Too much gain lowering the gain Swinging less than one cycle Too little gain increasing the gain Swinging in one cycle Reasonable gain achieved 32 16

17 Sluggish Response (contd.) Tuning procedure: 2. Adjusting Integral Action Visual Loop Tuning (6) Place the controller in manual mode, shorten the value of I m, then step out the CO at a reasonable value Immediately put the controller back in auto mode. Watch the process response to know what the controller action does Repeat the process until we get the ramps back to setpoint about half as fast as it moved away from setpoint form the CO step original original SP Response with reasonable gain (swinging in one cycle) SP To little I m (swinging at setpoint) Reasonable I m SP 33 Visual Loop Tuning (7) Tuning map for gain (P) and reset effect (I m ) Low gain (P) (wide PB) No reset (small repeat/min) (large min/repeat) Increasing reset (integral) action Fast reset (more repeat/min) (less min/repeat) Increasing proportional action High gain (P) (narrow PB) 34 17

18 The Effect of Adding Derivative Visual Loop Tuning (8) a) Best tuning achieved with proportional and integral modes only b) Too little derivative c) Derivative added gain increased integral action faster d) Too much derivative 35 What is Autotuning? Autotuning (also known as self-tuning) is a feature supplied by many controller, PLC and DCS vendors that allows the controller to tune itself It minimize the task of a control engineer in manually tuning the loops 36 18

19 Why Autotuning? The process is nonlinear or operated under widely varying conditions Need various combination of tuning parameters for different operating condition can be also accomplished by using operator s log The process characteristic change rapidly Frequent manual changing of the tuning parameters can not be expected to be able to produce satisfactory results The end user doesn t have the knowledge or experience for successful manual tuning 37 Autotuning Categories A variety of autotuning techniques found on the market: Scheduled tuning On-demand tuning On-line tuning 38 19

22 Relay Feedback Autotuning Method (contd.) The Procedure Forces the system to oscillate by a relay controller Require a single closed-loop experiment to find the ultimate frequency information No a priori information on process is required Switch relay feedback controller for tuning Find P crit and calculate τ crit according to the formula τ crit = On-demand Tuning (3) 4 d π a User specified parameter: d Decide d in order not to perturb the system too much Mostly use Ziegler-Nichols tuning rules for PID tuning parameters 43 On-line Tuning (1) The tuning parameters are determined by an auxiliary program that automatically evaluates the closed-loop behavior and calculates and modifies the tuning parameters whenever necessary Methods: Pattern recognition Others use a more formal mathematical procedure 44 22

23 On-line Tuning (2) Pattern Recognition Approach Example: The Foxboro EXACT TM (Expert Adaptive Controller Tuner) It observers the pattern of the response, then invokes a set of rules for determining new tuning parameters that will drive the pattern closer to a desired response pattern The technique: Does not require artificial load upsets instead it utilizes the normal process disturbances that occur; and Does not attempt to impose an arbitrary mathematical model on the process 45 On-line Tuning (3) The Foxboro EXACT TM Autotuning activated P 1 P 3 P 2 Control loop is using existing tuning parameters T New parameter calculated 46 23

24 Session Summary Manual tuning of PID controller can be conducted in various ways by means of some plant test There simply is no way to analytically tune a controller if we do not know the type of algorithm and the units Autotuning simplifies the tuning procedure of PID controller 47 24

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